a) Field of the Invention
The invention is directed to a process and an arrangement for measuring tapered internal thread, in particular on coordinate measurement devices using a one-ball or two-ball measuring method.
b) Description of the Related Art
It is suitable for the measurement of tapered internal and external threads. Two-ball measuring methods and arrangements for carrying out these methods which are applied in combination with a single-coordinate measurement device are known. For this purpose, two measuring clips with measuring balls are arranged at measuring spindles of the measuring device and brought into contact in opposite thread turns of an internal thread, the test piece being held on a floating measurement table of the measuring device. However, such methods have so far been applied only to cylindrical internal threads (DE-GM 87 11 490). Further, it is known from this reference to provide a single-angled or double-angled measuring clip to the measuring spindle for feeling the test piece, this measurement clip having two arms, each of which is provided with a measuring ball. The measuring balls are fastened at the arms opposite one another in the measuring direction and preferably have a defined spacing from one another. These measuring balls are brought into contact with opposite thread flanks.
The flank diameter of a tapered thread is defined in a testing plane (normal to the cone axis) which has a determined spacing from the end face of the thread (DIN 158; DIN 2999).
The ratios in cylindrical internal threads cannot readily be transferred to the measurement of tapered internal threads. Thus, in manufacturing, this is restricted as far as possible to summary testing of fundamental quantities by means of gauges. However, with gauges, which also include thread rings with tapered internal thread, the individual fundamental quantities of the respective thread must be determined. These measurements pose acute measuring problems. The three-wire method is often used for measuring tapered external threads (W. Langsdorff, "Messen von Gewinden [Measurement of Threads]", Springer Verlag, Berlin, Heidelberg, New York 1974, pages 69 to 74, and H. Zill, "Messen und Lehren im Maschinen-und Feingeratebau [Measurement and Gauges in Machine Building and Precision Instrument Building]", B. G. Teubner Verlagsgesellschaft, Leipzig 1956, pages 183 and 184).
As concerns the construction of the gauges, however, the user must decide what quantities or defining pieces are to be measured on the test pieces. If all fundamental quantities of the thread must be tested with respect to their combined action, tapered thread ring gauges or plug gauges must be used. But these ring gauges or plug gauges must themselves be measured with the necessary accuracy and the dimensional accuracy of such thread gauges must be tested after a certain period of use, which up to this point has been very involved and costly or simply inaccurate. For example, a ring gauge with a cylindrical thread was screwed into a tapered internal thread and the value of the flank diameter was derived from the screw-in depth. However, knowledge of the flank diameter and/or deviations from the angle of taper is essential for assessing a thread gauge.